Abstract
The eastern marginal fault zone of the Nara basin consists of a series of east-dipping reverse faults along the topographic boundaries between mountains and hills on the east and basins and lowlands on the west. This fault zone poses serious seismic hazards to the central Kinki area, especially the ancient capital Nara. However, the probability of earthquake occurrence in the next 30 years was very loosely constrained as 0-5 %. Since the 2019 fiscal year, we have conducted geologic investigations of the fault zone under a 3-year research project, “Comprehensive Research Project for the Nara-bonchi-toen Fault Zone.” In this project, we conducted paleoseismic trenching, pit excavations, borehole drilling, and tectonic geomorphic analyses along the Obitoke fault and Tenri flexure in the northeast Nara basin. The timing of the most recent faulting event on the Obitoke fault and Tenri flexure was estimated at sometime after ca. 4.5 ka and sometime after ca. 8.5 ka, respectively, younger than the previous estimate adopted by the current long-term evaluation of the earthquake occurrence by the Headquarters for Earthquake Research Promotion. The amount of the vertical displacement associated with the most recent faulting event on the Tenri flexure is estimated at 1-2 m based on the heights of tectonic scarps on young terrace surfaces. Geologic cross sections constructed by previous and newly-obtained borehole data led to the calculation of the vertical slip rate of ≥0.2 mm/yr for the Tenri flexure. The recurrence interval of the Tenri flexure is estimated at less than 5000-10000 years.
